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Broad area (Al,In)GaN laser diodes (LDs) are suitable for high optical output power in the near UV to blue spectral range. But for ridge widths larger than a few micrometers, the occurrence of filamentation is well known. We present experimental evidence that the single filaments tend to be phase-locked with defined phase offset and build up a so called supermode. Depending on driving current a coherent or incoherent superposition of different supermodes can be observed, which has a significant impact on the corresponding lateral far-field pattern. By a simulated reconstruction of the lateral mode profile of the laser mode propagating in free space we retrieve the field and phase distribution of the laser mode in the waveguide. In this context the coupling mechanism is discussed and the mode behavior is compared to supermodes in GaAs laser diode arrays.